This invention relates to the construction of thermal ink-jet printheads.
An ink-jet printer includes one or more cartridges that contain a reservoir of ink. The reservoir is connected by a conduit to a printhead that is mounted to the body of the cartridge.
The printhead is controlled for ejecting minute droplets of ink from the printhead to a printing medium, such as paper, that is advanced through the printer. The ejection of the droplets is controlled so that the droplets form images on the paper.
In a typical printhead, the ink droplets are expelled through orifices that are formed in an orifice plate that covers most of the printhead. The orifice plate is usually electroformed with nickel and coated with a precious metal for corrosion resistance. Alternatively, the orifice plate is made from a laserablated polyimide material.
The orifice plate is bonded to an ink barrier layer of the printhead. This barrier layer is made from photosensitive material that is laminated onto the printhead substrate, exposed, developed, and cured in a configuration that defines ink chambers. The chambers have one or more channels that connect the chambers with the reservoir of ink. Each chamber is continuous with one of the orifices from which the ink droplets are expelled.
The ink droplets are expelled from each ink chamber by a heat transducer, such as a thin-film resistor. The resistor is carried on the printhead substrate, which is preferably a conventional silicon wafer upon which has been grown an insulation layer, such as silicon dioxide. The resistor is covered with suitable passivation and other layers, as is known in the art and is described, for example, in U.S. Pat. No. 4,719,477, hereby incorporated by reference.
To expel an ink droplet, the resistor is driven (heated) with a pulse of electrical current. The heat from the resistor is sufficient to form a vapor bubble in the surrounding ink chamber. The rapid expansion of the bubble instantaneously forces a droplet through the associated orifice. The chamber is refilled after each droplet ejection with ink that flows into the chamber through the channel(s) that connects with the ink reservoir.
In the past, the orifice plate and barrier layer were mechanically aligned and bonded together, usually in a high-temperature and high-pressure environment. Inasmuch as the orifice plate and barrier layers are made of different material, the need for precisely aligning these two components is complicated by the differences in their coefficients of thermal expansion. Also, this approach to constructing a printhead limits the minimum thickness of the bonded components to about 25 xcexcm, which thus prevents the use of very small droplet volumes with the attendant high resolution and thermal efficiencies such use would permit.
Currently, the notion of an integrally formed orifice plate and barrier layer has been considered. For clarity, an integrated orifice plate and barrier layer will be hereafter referred to as a droplet plate, which is a unitary plate defining both the ink chambers and orifices (the orifices hereafter referred to as nozzles). It will be appreciated that such a plate eliminates the problems associated with the orifice plate and barrier layer construction just mentioned.
Manufacture of such a droplet plate may be carried out using photolithographic techniques, which techniques generally offer a high degree of design latitude. It is desirable, however, to arrive at a simple, reliable fabrication process that has very precise dimension control as well as one that results in materials that are robust and inert.
The present invention concerns a process for fabricating a droplet plate and provides design flexibility, precise dimension control, as well as material robustness. Also provided is a droplet plate fabricated in accord with the process.
The process generally comprises a two-stage deposition and patterning/etching procedure whereby the firing chambers in the droplet plate are formed first, followed by the nozzles. The process does not rely on etch selectivity between materials. As a result, a good deal of design flexibility is provided in selecting the droplet plate material. In this regard, robust, highly inert materials can be used as the droplet plate to provide effective resistance to chemical attack, such as from the ink.
The deposition aspect of the process is preferably carried out using plasma-enhanced chemical vapor deposition (PECVD), which, among other things, permits the use of the highly inert materials (such as silicon oxide) as compared to, for instance, spin-on polymers and epoxies. Sputter deposition, also known as physical vapor deposition (PVD), may also be employed for depositing the dielectric material.
Although an integrated droplet plate (comprising both firing chambers and associated nozzles) is fabricated by the process of the present invention, the process steps are such that the firing chambers and nozzles may be shaped independently of one another.
In a preferred embodiment, the droplet plate is formed directly on the printhead substrate, which substrate carries the heat transducers as mentioned above. A dielectric material layer is deposited via PECVD onto the substrate and shaped to form firing chambers. In one approach, this shaping is carried out by depositing the layer to a depth matching that of the firing chamber and then employing reactive-ion-etching to define the chamber volume.
The chamber volume is then filled with sacrificial material, which is planarized before an additional amount of dielectric material is deposited to a depth desired as the thickness of the nozzle. The nozzle volume is then etched and the sacrificial material removed to complete the droplet plate fabrication.
In another embodiment, a single deposit of dielectric material is made over previously placed bumps of sacrificial material. The bumps are sized to match the volume of the firing chambers and are placed over each heat transducer. The layer is then etched to define the nozzles, and the sacrificial material is then removed, yielding a droplet plate that is produced with a single PECVD step.